Changes include:

• use the first Bianchi identity in the computation of the Riemann tensor
• compute volume forms with contravariant indices only as needed
• don't try to simplify trivial expressions consisting only of a single number or symbolic variable

Component: manifolds

Author: Marius Gerbershagen

Branch/Commit: 59691a8

Reviewer: Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/33110